The present invention relates generally to grain drying equipment and more particularly to an improved low profile continuous crossflow column grain dryer with optimum exhaust drying air recirculation through the burner to reduce fuel consumption, automatic preheat of fresh incoming grain, improved drying and cooling airflow volume and cfm/bushel vs blower horsepower by a variable grain column thickness design through both the drying zone and cooling zone, and precise external adjustment for control of grain flow to the metering rolls for optimum drying uniformity along the dryer length.
It is generally believed that continuous crossflow dryers, that is, those dryers which have wet grain continually or semicontinually entering the dryer and dried grain continually or semicontinually exiting (i.e. continuous "periodic unloading" of specific amounts of grain) the dryer, with drying air passing generally perpendicular through the flowing column of grain, were not suitable for drying grains having a high moisture content. The reason for the difficulties experienced in the use of conventional continuous crossflow dryers was that they only operated at their optimum design performance level over a fairly narrow range of moisture removal due to noncontrollable design conditions such as a fixed cooling airflow, and a fixed heated airflow, flowing through a constant width column or a constant depth of grain.
At a grain moisture removal of 6 to 8 percentage points, most conventional dryers work satisfactorily. The cooling rate is matched fairly well with the drying rate. The grain column is usually split 25-35 percent cooling and 65-75 percent heating. The total blower horsepower is normally split at between 30-40 percent cooling and 60-70 percent drying. Dryers with 25 percent cooling PG,3 column usually use the upper extreme in cooling horsepower, thus operating the cooling plenum at a higher static pressure than the heating plenum and delivering 50-100 percent more cool air per bushel than drying air.
Under conditions wherein grain coming from the field is very high in moisture, and the drying rate is slowed significantly, the grain in such prior art systems was over cooled, which is not a particular problem from the standpoint of the quality of the grain dried, but it does waste considerable energy. Under very dry grain inlet conditions wherein the moisture removal is in the 3-5 percentage range, the grain flow rate is high and cooling is inadequate. If grain conditioned by such a process is to be stored in a non-aerated storage and therefore has to be cooled considerably after being dried in the dryer, the only reasonable solution was to significantly reduce the drying temperature thereby drastically slowing the drying rate to the point where the grain retention time in the cooling zone was adequate to cool the grain. It is well known that the efficiency of the drying process is reduced and the fuel cost per bushel increased considerably when the plenum operating temperature of a crossflow dryer is significantly reduced. It is also well known that the grain to be stored in non-aerated storage cannot be too warm or it will deteriorate rapidly.
Another weakness of most conventional continuous crossflow column grain drying devices is that when drying grains under conditions where cooling the grain in the dryer is not desired, the cooling airflow must be blocked off and the cooling grain column is of little or no value in drying. There is, therefore, a need for improved equipment of this type which will adequately compensate for this situation by having a design that can be easily adjusted to provide drying of grain in the grain column area normally used for cooling to maximize the performance and capital investment of the dryer, and to further increase the efficiency of the dryer by providing suitable means for controlling and recycling the very dry high quality air from the lower zone of the grain column (the cooling zone when dryer is used for cooling) while yet being able to differentiate between and return the exhaust air that is suitable and reject the exhaust air that is unsuitable whether this differentiation should occur (1) within the lower zone, (2) between the lower zone and the lower portion of the upper zone, or (3) within the lower portion of the upper zone.
It is also known that conventional crossflow dryers are normally: (1) of a full pressure design, using positive pressure in a heating as well as a cooling plenum, or (2) designed with suction cooling and pressure heating. But, they do not in one common structure embody the capability to perform in either method with quick and easy adjustment between methods, a management capability thought to be highly desirable, especially in farming regions where grain sorghum (milo) or sunflowers (both of which are crops with abnormally high combustible seed coat particles which accumulate in the heat plenums of dryers that recirculate the suction cooling air through the burner, causing fires in the dryer plenum or grain column) plus other cereal grain crops are grown in one farming operation. Having a dryer capable of being easily converted in a matter of a few minutes would allow full pressure heating and cooling of milo, sunflowers, or other crops with flammable residue, to be dried with the pressure heating and cooling mode of operation using ambient air with no recycled exhaust air. Then, when it is desirable to dry corn, wheat, soybeans or other "safer" crops with residue that is considerably less flammable, the dryer mode can be adjusted for suction cooling plus recycling of the less humid portion (approximately half) of the exhaust heated air without time consuming dryer modifications, thus, reducing fuel consumption by 35 to 50 percent of the fully pressurized heat and cool process fuel costs, while not significantly affecting drying capacity. In some farming and elevator operations, switching drying modes may take place several times per week during the periods when several types of grain are brought in to be dried.
In addition to easy adjustment and control of the direction, volume, and quality of the airflow when changing grains being dried, it is also desirable to adjust the flow profile of the grain as it enters the metering rolls due to the extreme differences in grain density, shape, size, and frictional surface characteristics, relative drying rates, plus trash and foreign material in the grain. For example, soybeans generally have a density of approximately 60 lbs. per bushel, and are relatively difficult to dry. Corn weighs about 56 lbs. per bushel and varies widely in shape from flats to rounds causing considerable variations in airflow and static pressure, and is relatively difficult to dry. On the other extreme, sunflowers, quite often grown on the same farm with corn, soybeans, or wheat, weigh from 24-32 lbs. per bushel but in some years, may be immature and weigh as low as 16-18 lbs. per bushel; sunflowers dry very rapidly at much lower air temperatures and are very light and bulky, thus require a much thicker grain flow to the metering rolls for proper handling rates to avoid extensive modification of the metering drive train which would cause considerable time loss and inconvenience to the operator.
In conventional drawings a uniform common heat level is used throughout the plenum chamber which is lower than desired in the upper portion of the drying column where the grain is wettest, and higher than desirable where the grain exits the drying zone.
There is, therefore, a need for a continuous flow drying apparatus which will overcome the aforementioned problems found with prior art devices.